Devices capable of detecting light, commonly referred to as photodetectors, are important in many areas of modern technology. For example, they are essential components of optical communications systems. Such systems have a light source and a photodetector optically coupled to each other by means of a glass transmission line which is commonly termed an optical fiber. Information is transmitted through the fiber in the form of light pulses. In such systems, as well as in other applications, it is desirable to have highly sensitive photodetectors because such photodetectors, for example, permit longer distances between the light source and photodetector thus requiring, e.g., fewer repeaters for the system. Of course, highly sensitive photodetectors are useful in many other applications. For example, optical time domain reflectometers are used to measure some characteristics of glass transmission lines.
The ultimate photodetectors, in terms of quantum efficiency, would be a detector capable of detecting a single photon. However, many types of photodetectors are not even theoretically capable of detecting single photons. For example, a p-i-n photodiode, which is commonly used in optical communications systems, has no gain and thus, the absorption of a single photon results in only a single electron-hole pair which must be detected by external circuitry. However, the dark current that arises from the thermal generation of carriers is sufficiently large to mask a single electron-hole pair. The limitation in sensitivity imposed by the one-to-one correspondence between the number of absorbed photons and the number of electron-hole pairs can be avoided if the photodetector exhibits gain, i.e., more than one electron-hole pair ultimately results from each absorbed photon.
Accordingly, much effort has been directed toward developing photodetectors that exhibit gain. One such photodetector is the avalanche photodiode. In such photodiodes, carriers impact ionize to generate more carriers which in turn ionize, etc., thus lending to avalanche multiplication. Such devices are operated at reverse bias voltages that are sufficiently high to ensure that avalanche multiplication, leading to gain, occurs. Above a certain voltage, commonly termed the breakdown voltage, the multiplication becomes extremely large.
However, there are several reasons why avalanche photodetectors are not easily used to detect small numbers of photons. For example, there is also a dark current present which results from the necessity of biasing the device. There are variations in the magnitude of the dark current and these variations are typically sufficiently large, at least when the device is operated at noncryogenic temperatures, so that the additional current resulting from the absorption of the single photon is undetectable. It should be remembered that the dark current also undergoes avalanche multiplication and that the size of the dark current typically increases as the energy bandgap decreases.
However, attempts have been made to use avalanche photodetectors in situations where they are capable of detecting small numbers of photons, i.e., photon counting. These attempts have used a time varying potential, i.e., a gated potential, periodically above breakdown to achieve high sensitivity. For example, U.S. Pat. No. 4,303,861 issued on Dec. 1, 1981, describes one such photon counting system. The system described operated the photodetector at a bias in excess of the avalanche potential, i.e., at a bias above the breakdown voltage. The photodetector was a Schottky barrier diode. The system was cooled to cryogenic temperatures so that the avalanche process occurred only when photons were absorbed. Upon detection of the avalanche, the bias voltage was reduced to a value below the avalanche potential to quench the avalanche.
However, for most applications, room temperature operation is desired. This is difficult, especially at relatively long wavelengths, because of the size of the dark current. Room temperature operation and sensitivity at long wavelengths would lead to a device that might have many applications as, for example, in the optical time domain reflectometer previously mentioned. This is an instrument which sends an optical pulse into an optical fiber and measures the time until a reflection returns. This measures, potentially with high precision, the length of the fiber together with other transmission line characteristics such as splice losses.
Long wavelength avalanche photodetectors often use Group III-V compound semiconductors because of their relatively low bandgaps. However, such semiconductors appear undesirable for photon counting avalanche phtodetector operation at room temperature because of the thermal noise resulting from the low bandgap. Additionally, such devices typically are heterojunction devices with separate absorption and multiplication regions. It might be expected that the interface might have many traps which would also make room temperature operation impossible.